Bismuth halide vapor laser

ABSTRACT

The low temperature operation, i.e., between 300° and 500° C, of a bismuth halide vapor laser avoids the formation of the metal dimer Bi 2  the occurrence of which in the operation of a pure bismuth laser significantly reduces the gain and stimulated output due to absorption of the laser energy. The successful operation of a bismuth halide laser provides the basis for developing a laser operating in the underwater transmission band of between 4,700 and 4,800 Angstroms which lies near the minimum of the sea water absorption scattering curve thus rendering it a prime candidate for underwater applications.

BACKGROUND OF THE INVENTION

The concept of producing pulsed metal vapor laser transitions by placinga metal halide of the desired metal within an enclosure and vaporizingthe metal halide to provide a metal halide vapor is described in detailin U.S. Pat. No. 3,934,211, issued Jan. 20, 1976, entitled "Metal HalideVapor Laser", and assigned to the assignee of the present invention.This U.S. patent is incorporated herein by reference. There is furtherdisclosed in U.S. Pat. No. 3,936,772 issued Feb. 3, 1976, entitled "HighFlow Metal Halide Vapor Laser", a technique for obtaining high pulserate metal vapor laser transitions. This latter patent is assigned tothe assignee of the present invention and is also incorporated herein byreference.

In accordance with the above-identified U.S. Pat. No. 3,934,211, themetal halide vapor thus produced is thereafter disassociated to provideground state metal atoms of sufficient number density to create acondition for resonance trapping and substantially simultaneoustherewith, ground state metal atoms are excited to an upper laser level,while maintaining a sufficient number in the ground state to preservethe resonance radiation trapping condition, with electrons sufficientlyenergized to create a population inversion between the upper and lowerlaser level. The excited metal atom is permitted to emit laser radiationby stimulated emission to a lower laser level and the emission isradiated, preferably between a pair of externally mounted mirrors. Themetal atom is permitted to relax from the lower laser level to theground state and the aforementioned steps are repeated. U.S. Pat. No.3,934,211 identifies as well known the metal vapor lasers of copper,manganese, and lead.

As disclosed in U.S. Pat. No. 3,934,211, it was determined that themetal component of a metal halide molecule can be made to lase attemperatures substantially below those required for pure metal vapors.It was further determined that thermal energy at or below those normallyemployed in pure metal vapor lasers do not provide adequate atomicdensitites and, therefore, it is necessary to provide collisionalexcitation energy with energetic electrons to obtain dissociation of themolecular vapors.

While pure copper, lead, and manganese readily exhibited the operationalcharacteristics of desired metal vapor laser transitions, thus renderingthem prime candidates for modification to produce corresponding metalhalide vapor lasers suitable for operation at lower temperatures,attempts to achieve pure bismuth metal vapor laser transitions provedunsuccessful. The lack of success of achieving successful operation of apure bismuth metal laser is attributed primarily to the fact that themetal dimer, or diatomic molecule, Bi₂ is formed by the pure bismuthmetal laser and the metal dimer Bi₂ absorbs substantial laser energy atthe preferred underwater transmission wavelength of 4722 Angstromthereby destroying the laser gain required to support the stimulatedlaser output.

The use of pure bismuth vapor to produce laser action at 4722 Angstromshas been unsuccessful to date because of a fundamental problem inbismuth volatilization, namely the formation of the diatomic moleculeBi₂ in the vapor in addition to atomic Bi. For instance, at 1200° K theratio of molecular Bi₂ to atomic Bi vapor pressure is about 1 to 3. Thusat the temperatures required to achieve even the minimum Bi densitiesfor laser action, greater than 25% of the particles are in the molecularform. These molecules absorb electrical excitation energy at the 4722Angstrom atomic radiation, thus rendering pure bismuth metal a poorcandidate for lasing.

Inasmuch as the pure bismuth metal laser failed to achieve theoperational status of the pure copper, manganese, and lead lasers, ithas been generally concluded that the lower operating temperaturebenefits realized by the addition of metal halide molecules as taught byU.S. Pat. No. 3,934,211 was of no benefit to the pure bismuth laser.

SUMMARY OF THE INVENTION

Detailed studies of the operation of bismuth halide lasers resulted instrong emissions at 4722 Angstroms from fast pulse discharges in bismuthiodide, BiI₃, vapors at 325° C. The testing of this bismuth halideformed by the combination of the pure bismuth metal and iodide as themetal halide molecule substantiated the conclusion that the upper laserlevel 7s⁴ P_(1/2) of Bi can be efficiently excited in a BiI₃ vapor byelectron impact excitation. Furthermore, it was observed that only theradiation lines of Bi were present in fluorescence between 2800Angstroms and 6000 Angstroms thereby indicating that the diatomicmolecule Bi₂ was neither present nor excited in the discharges.

The absence of the metal dimer or diatomic molecule Bi₂ in theevaluation of bismuth halide lasers including BiI₃, BiBr₃, and BiCl₃supports the previously unexpected conclusion that practical usefullasers can be developed through the use of bismuth halide metal vaporseven though the pure bismuth metal vapor is ineffective as a practicallasing composition.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the followingexemplary description in connection with the accompanying drawings:

FIG. 1 is a plot of bismuth and bismuth halide vapor curves; and

FIG. 2 is a plot of the energy level diagram of the bismuth.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the above referenced U.S. Patents describe in detail the logicalextension of operative pure metal vapor lasers to that of a metal halidevapor laser to permit lower temperature operation, the inability tosuccessfully achieve lasing with a pure bismuth vapor laser, due to theadverse energy absorption by diatomic molecule Bi₂, has essentiallyeliminated consideration of a bismuth halide lasing composition.

The invention disclosed herein documents detailed studies of thecharacteristics of a bismuth halide vapor laser with the conclusionbeing that the operation of a bismuth halide vapor laser consistent withthe teachings and structures of U.S. Pat. No. 3,934,211, which isincorporated herein by reference, does indeed effect desired lasingaction while avoiding the formation of the diatomic molecule Bi₂.

It has been observed experimentally that the vaporizing or heating of asolid bismuth halide composition, such as BiBr₃ (s) produces the gaseousbismuth halide composition BiBr₃ (g) and does not produce the elementalconstituents Bi and Br. Inasmuch as substantially all the bismuth iscontained in the gaseous composition BiBr₃ (g), the bismuth vaporpressure is low, thus supporting a conclusion that the diatomic moleculeBi₂ is not present. The absence of the diatomic molecule Bi₂ eliminatesthe adverse energy absorption factor which has rendered the pure bismuthvapor laser inoperative. Similar results, namely the generation of agaseous bismuth halide composition free of the diatomic molecule Bi₂ hasbeen achieved by heating solid bismuth halide compositions includingBiI₃ and BiCl₃.

The contrast between the vapor pressures of pure bismuth and bismuthhalides is apparent from the curves of FIG. 1 wherein the bismuthhalides have vapor pressures approximately 10 Torr at 250° C while thepure bismuth metal exhibits a vapor pressure of approximately 10 Torr at1100° C.

Of special interest is the pulse shortening observed at the 4722Angstroms line in BiBr₃ and BiI₃ pulse discharges. Although thisradiation line is very close to the optimum underwater transmissionwavelength, laser action for this radiation line has not been previouslyobserved in pure bismuth metal systems due to the formation of theabsorbing species represented by the diatomic molecule Bi₂. The absenceof Bi₂ radiation in the bismuth halide discharges indicates that 4722Angstrom laser action is available from the bismuth halide vapor laserdespite the fact a pure bismuth metal laser is inoperative. Results ofthese studies and observations further supports the observation thatother metal laser systems not operative as a pure metal laser may indeedbe operational as a metal halide vapor laser.

Strong emission at the 4722 Angstrom wavelength has been observed fromfast pulse discharges in BiI₃ vapors at approximately 325° C. The energylevel diagram of FIG. 2 indicates that the upper laser level 7s⁴ P_(1/2)of bismuth can be efficiently excited in a BiI₃ vapor by electron impactexcitation. Furthermore, only the radiation lines of Bi were observed influorescence between 2800 Angstroms and 6000 Angstroms therebyindicating that Bi₂ was not present or excited in these discharges. Theupper laser level of bismuth is a resonance state which is about 4 eVabove the ground state 6p³ 4 S_(3/2), and the terminal laser level isthe metastable state 6p³ 2 D_(3/2) which is about 1.4 eV above theground state.

The dissociation energies for the various bismuth halide vaporcompositions are approximately 2.5 eV for BiI₃, and approximately 3 eVfor BiBr₃ and BiCl₃. Bismuth halide vapors at temperatures below 300° Ccontain very few free Bi atoms in the absence of electricaldissociation.

The preferred operating temperature for the bismuth halide laser,consistent with the curves of FIG. 1, is between 300° C and 500° C.

During the first few nanoseconds of a pulsed discharge in the bismuthhalide vapors, the radiation characteristic is predominantly the bismuthresonance radiation and the radiative lifetime of the upper laser levelis determined by the resonance radiation. This lifetime is relativelyshort, i.e., 2.8 nanoseconds. An imprisonment of the resonance radiationoccurs at increased dissociation levels and the upper laser levellifetime is extended to the radiative lifetime of 4722 Angstromsradiation, i.e., approximately 111 nanoseconds. With this extendedlifetime, the upper laser level of the bismuth halide vapors can bepumped by conventional fast pulse circuitry. This dissociativeexcitation by electron impact along with resonance trapping produces apopulation inversion in the bismuth halide vapor laser consistent withthe teachings and disclosure of the copper halide vapor laser of theabove-referenced U.S. Pat. No. 3,934,211.

While the above disclosure recognizes the preferred usefulness of thebismuth halide vapor laser at the laser radiation level of 4722Angstroms for use in underwater applications, it is clearly apparentthat all possible lines radiated from the various resonance levels tothe metastable levels represent potential laser action. Multiple laserwavelengths can be realized through proper design of an optical cavitysystem.

A further advantage realized from the use of bismuth halide vapor lasersis that the deposition of pure bismuth on the envelope walls or windowsof a laser apparatus following thermal or electrical dissociation can beremoved by "scouring" of the bismuth atoms by the halogen particles.Thus the original laser species is recovered and degradation of theoptical windows of the laser apparatus due to bismuth condensation canbe avoided.

We claim as our invention:
 1. In a laser apparatus for obtaining metalvapor laser transitions at temperatures substantially below the metalvaporization temperature, the combination of:an enclosure, a bismuthhalide selected from the group consisting of BiI₃, BiCl₃ and BiBr₃within said enclosure, means for vaporizing said bismuth halide byheating said bismuth halide to a temperature between approximately 300°C and 500° C while avoiding the formation of the metal dimer Bi₂, meansfor dissociating the bismuth halide vapor for providing ground statebismuth metal atoms of sufficient number density to create resonanceradiation trapping, means for creating a population inversion between adesired upper laser level and a lower laser level of the bismuth byexciting ground state bismuth atoms with energized electrons and formaintaining a sufficient number of bismuth atoms in the ground state topreserve said resonance radiation trapping, and means for stimulatingthe emission of a beam of radiation from the inverted medium.
 2. In alaser apparatus as claimed in claim 1 wherein said beam of radiation isof a wavelength between 4700 Angstroms and 4800 Angstroms.
 3. In a laserapparatus as claimed in claim 1 wherein said beam of radiation is 4722Angstroms.
 4. In a laser apparatus as claimed in claim 1 wherein saidupper laser level corresponds to the resonant state and said lower laserlevel corresponds to the metastable state.
 5. In a laser apparatus asclaimed in claim 1 wherein said upper laser level is 7s⁴ P_(1/2) andsaid lower laser level is 6p³ 2 D_(3/2).
 6. In a laser apparatus asclaimed in claim 1 wherein the dissociation energy for BiI₃ isapproximately 2.5 eV and the dissociation energy for BiBr₃ and BiCl₃ isapproximately 3 eV.
 7. In a laser apparatus as claimed in claim 1wherein the vaporizing of BiI₃, BiBr₃ and BiCl₃ produces the gaseousbismuth halide compositions BiI₃ (g), BiBr₃ (g) and BiCl₃ (g)respectively without the formation of the elemental constituents of therespective bismuth halides.